1. Field of the Invention
The invention relates to digital-to-analog converters (DAC), and more particularly to automatic gain control in DACs.
2. Background Art
A digital-to-analog converter (DAC) is an electronic circuit that receives an n-bit digital word from an interface circuit and generates an analog output that is proportional to the received digital word. Digital codes are typically converted to analog voltages by assigning a voltage weight to each bit in the digital code and then summing the voltage weights of the entire code.
DACs can be designed for a wide range of applications, including general data acquisition applications and special applications, such as, but not limited to, video or graphic outputs, high definition video displays, ultra high-speed signal processing, and digital video recording.
The major factors that determine the performance quality of a DAC are resolution, speed, and linearity. Resolution refers to the smallest change in the output analog signal that is supported. The resolution determines the total number of digital codes, or quantization levels, that will be recognized by a converter. The speed of a DAC is determined by the time it takes to perform the conversion process. Measurement accuracy is specified by a DAC's linearity. Integral linearity, which is also referred to as relative accuracy, is a measure of the linearity over the entire conversion range. It is often defined as the deviation from a straight line drawn between the endpoints and through zero (or offset value) of the conversion range. Differential linearity is the linearity between code transition.
The accuracy of a DAC is critical in certain applications. When a DAC has a high degree of accuracy, the control range of a subsequent gain control loop and complexity of circuit stages that may follow a DAC can be reduced. As a result, system cost and power requirements can be minimized.
When Bi-CMOS technology is used to implement a DAC, at least three factors impact the accuracy of the DAC. First, resistive loads of resistors used in a DAC can vary depending on temperature and fabrication process. Second, the current through differential pairs of transistors within a DAC can vary based on fabrication process, temperature and supply voltage. Third, when bipolar devices are used there is some current flow into the base of the devices due to finite beta (β) values.
What are needed are systems and methods to reduce the impact of the above variations to improve DAC accuracy.